In a conventional semiconductor integrated circuit device, silicon dioxide films for isolation (field isolation insulating films) are formed both on a memory cell area having a plurality of memory devices (elements) and on an environmental device area (e.g., peripheral area) having a plurality of devices (elements) using the same local oxidation technique. Consequently, for realizing an extremely fine memory cell area, i.e. a finely defined small-sized memory cell area, how the major obstacle that must be considered is lateral extension (bird-beak) of the silicon dioxide film, generated by the local oxidation, can be made smaller.
However, as a result of increasing the impurity concentration of the semiconductor substrate to make smaller the bird beaks and to make further fine the devices, the kink has developed in the subthreshold characteristic of the MOS transistors due to the leakage current flowing through channels which are parasitically formed at the boundary between the active region and the field isolation area. This exhibits such an apparent characteristic that the MOS transistors having low threshold voltages are connected in parallel to each other. The phenomenon of the kink is observed even in the device having the wide channel, whereas in the device having the narrow channel width, it appears in the form of a lowering of the threshold voltage.
In order to suppress the generation of the kink, there has been proposed a technique, for example, in Japanese Laid-open No. hei 2-237158, wherein the generation of the kink current (the current which is allowed to flow by the above-mentioned kink phenomenon) is suppressed by locally increasing the impurity concentration at each end of the device through which the kink current flows.
This technique will be described with reference to FIG. 10. Here, a trench type isolation is used, that is, a field oxide 2 is buried in a trench formed in a field isolation area. Also, a thick polycrystalline silicon film 19, which serves as a mask in boron (B)-ion implantation, is formed on an active area through a thin oxide film 3. Boron-ions are implanted by a specified depth perpendicularly to a semiconductor substrate 1. At this time, by the effect of lateral scatter, p-type layers 15 having a higher concentration than the semiconductor substrate 1 are formed on the ends of each device. Also, a p-type channel stopper layer 20 for isolation is formed under the field oxide 2.
Other measures for overcoming the kink have been proposed, for example, in Japanese Laid-open hei 2-303049, wherein the side walls of the field isolation trench formed on the edges of the active area where the gate electrodes are crossed to each other are inclined. This technique will be described with reference to FIG. 11. Here, in forming the field isolation trench on the surface of the semiconductor substrate by dry etching, energy beams for excitation are scanned in a specified direction, so that side surfaces 23, in the above specified direction, of the field isolation trench are made to be perpendicular to the surface 21 of the semiconductor substrate, and the other side surfaces 22 of the field isolation trench are inclined. By formation of a gate electrode 18 so as to be crossed to the inclined surfaces, the generation of the kink is suppressed. In this figure, numeral 24 indicates the bottom surface of the field isolation trench. An insulator is buried within the field isolation trench.
Meanwhile, in the device which has the field isolation insulating film of the short bird beaks and is narrow in channel width, even if the kink is not generated, the whole area of the device becomes the kink current generation area, and accordingly, there develops a problem in which the threshold voltage is reduced by the so-called inverse narrow-channel effect, as shown in FIG. 13(a).
In the above conventional techniques, the former has the following disadvantages: Namely, as shown in FIG. 10, the impurity concentration of the p-type layer at the channel edge is locally increased, and consequently the kink current is suppressed: however, the junction characteristic between the source and drain diffusion layer and the semiconductor substrate 1 is deteriorated, which brings about the reduction in junction breakdown voltage and the increase in junction leakage current, thereby deteriorating the data retention ability and increasing the stand-by current.
For achieving the high integration of the semiconductor integrated circuit, the impurity concentration of the semiconductor substrate 1 must be increased. However, along with this, it is required to increase the impurity concentration of the p-type layer 15 at the channel edge. Consequently, the above junction characteristic is further deteriorated.
Also, in the latter one of the discussed conventional techniques, the inclination of the side surfaces of the field isolation trench is controlled by scanning the energy beams for excitation in the specified direction. Accordingly, the side surfaces 23, in the specified direction, of the side walls of the trench are necessarily perpendicular to the surface 21 of the substrate, which makes it impossible to take the perfect measure. The reason for this is that the semiconductor integrated circuit device is constituted of a plurality of devices, and gate electrodes 18 in the devices are not necessarily directed in the specified direction. Accordingly, even if the gate electrodes 18 in some devices are crossed to the perpendicular surface, in the other devices, a kink is liable to be generated. This problem is solved by a method of specifying the direction of the gate electrodes of all of the devices; however, in this method, the dimension of the chip is increased due to the restriction of the lay-out, thereby making it difficult to realize this method.
Also, the above devices being narrow in channel width which causes the inverse narrow-channel effect is disadvantageous in that, since they are mainly formed in the memory cell area, the off-current (current flowing when the gate voltage is not applied) of the MOS transistors is increased along with the reduction in the threshold voltage, which brings the deterioration of the memory retention characteristic and the increase in the stand-by current.